Exhaled nitric oxide (NO) is a promising non-invasive tool to assess lung function, particularly in inflammatory diseases such as asthma. Traditional techniques, such as spirometry, are aimed at examining physical features of the lungs, such as airway caliber. In contrast, exhaled NO is derived from the lung tissue, and thus potentially provides important new physiological and clinical information. NO exchange occurs in both the airways and alveolar regions. Therefore, exhaled concentration alone depends strongly on exhalation flow rate and does not adequately reflect the rich features of NO exchange. We describe NO exchange with a two-compartment model (alveolar and airway compartment) and several "flow-independent" parameters: the steady state alveolar concentration, the airway diffusing capacity, the maximum airway flux, and the airway tissue concentration. In contrast to exhaled concentration alone, these features of NO exchange are inherently specific to the anatomical regions of the lungs, and do not depend on the exhalation flow rate. Our preliminary data (as well as data from other groups) demonstrate the potential of these parameters in providing useful clinical and physiological information. However, the precise interpretation and the factors that impact these parameters are poorly understood. We hypothesize that endogenously produced NO appearing in the exhaled breath can be used to probe metabolic features of both the airway and alveolar regions of the lungs. Our specific aims are geared toward improving our understanding of NO exchange dynamics in healthy adults and children, specifically examining the impact of the following features of pulmonary gas exchange on the characterization of NO exchange dynamics: 1) lung NO metabolism brought about by inhalation of monodisperse aerosols of L-Arginine (substrate for Nitric Oxide Synthase, NOS, to stimulate NO production), and L-NMMA (competitive inhibitor of NOS to reduce NO production); and 2) axial diffusion in the gas phase brought about by manipulating the physical properties of the insufflating gas (e.g., Heliox) and breathhold time. The proposal combines human subject experiments with advanced mathematical modeling to enhance our understanding of NO exchange dynamics. Completion of the specific aims will provide the next level of understanding necessary for exhaled NO to be fully utilized as a research and clinical tool.